Gas turbine engines operate by directing a flow of combustion gases through alternating arrays of stationary and rotating blades. The combustion gases which perform the work in the engine are generated in the combustor by burning fuel in the presence of compressed air.
Proper combustion requires good dispersion or atomization of the fuel in the combustor to ensure proper mixing with the compressed air. This efficient atomization of fuel must be effective over a wide range of fuel flow rates which reflect the wide range of engine operating conditions.
At the relatively high fuel flow rates which occur during most engine operating conditions, a significant pressure differential exists across the outer wall of the combustor. This significant pressure differential and the relatively high flow rates can be utilized to achieve a desirable atomization of the fuel. However, at extremely low engine operating speeds, such as the speeds occurring during start-up conditions, the fuel flow rates are low and the pressure differential across the wall of the combustor is also low. Consequently it has been difficult to obtain the required atomization when the engine is being started.
Many attempts have been made to develop an apparatus to achieve the required fuel atomization during a wide range of engine operating conditions. One such apparatus has been a duplex fuel injector mounted in the wall of the combustor. The duplex fuel injector includes a very small primary fuel aperture and a larger secondary fuel aperture both of which are in the combustor. The primary fuel aperture typically has a diameter of between 0.01 inch and 0.10 inch. Both the primary and secondary fuel apertures are in communication with a flow divider valve which is operable to selectively atlernate flows between the primary and secondary fuel apertures. More particularaly, the flow divider valve is operative to direct fuel to the primary fuel aperture during engine start-up conditions. The rate of flow of fuel during these conditions is relatively low, in accordance with the engine operating requirements. However, since the primary fuel aperture is very small, the required atomization can be achieved even with low fuel flow rates. As the engine operating speeds and fuel requirements increase, the flow divider valve will change modes to direct fuel from the primary fuel aperture to the larger secondary fuel aperture. As noted above, the higher fuel flow rates occurring in during most post start-up conditions, combined with the pressure differential across the combustor wall, will enable an efficient atomization of fuel.
Although the above described duplex fuel injector is theoretically functional, several problems have been encountered during the actual use of the apparatus. More particularly, the prior art duplex fuel injector is mounted such that both the primary and secondary fuel apertures are within the engine combustor. The combustion processes taking place within the combustor generate very high temperatures (e.g. 4,000.degree. F.) and generate various combustion by-products. As a result of these extreme conditions, the very small primary fuel aperture is likely to become at least partly blocked due to coking or varnishing. This blockage of the primary fuel aperature may cause significant problems in connection with the starting of the engine.
In view of the above, it is an object of the subject invention to provide an apparatus for achieving a desirable atomization of fuel across a wide range of engine operating conditions.
It is another object of the subject invention to provide an apparatus that enables the use of a fuel injector in the combustor that is not likely to be blocked by coking.
It is an additional object of the subject invention to provide an apparatus that can achieve adequate atomization at low fuel flow conditions without employing a small fuel aperture in the combustor.
It is another object of the subject invention to provide an apparatus that is spaced from the combustor, and that is operative to create an efficient atomization of fuel during engine start-up conditions.